For example, the magnetic field emitted from a human body such as that from brain or heart contains many important real-time biological pieces of information. When the cardiac magnetic field detected by a MCG detector such as 64 channel SQUID gradiometers, the electrophysiological function of heart can be two-dimensionally mapped. It is furthermore possible to obtain overwhelmingly more accurate and more diverse diagnostic pieces of information than those available by the method based on waveform analysis of electrocardiograph, such as time and space information of current vector flowing along the irritation conduction system.
Acute myocardiac infarction which is an epitome of ischemic heart diseases is believed to be one of the three major causes of death of Japanese. Expensive high-tech medical technologies are used for the cure thereof. If more accurate and more rapid diagnosis is possible, it would bring about considerable effects not only in the remarkable reduction of medical cost but also in life-saving.
Partly because of the lapse of only a short period of time from development of the magnetocardiograph, its popularization is still in an initial stage in spite of these rich potentialities.
One of the causes of this slow progress of popularization lies in a chamber type magnetic shield room made of permalloy which is expensive and inconvenient. The magnetic screening performance is not required to be so high as that required for cerebral magnetic field measurement, but here is an increasing demand for a flexible magnetic shield apparatus which permits measurement by bringing the patient together with his or her bed.
In other words, there is a demand for development of a separate movable type high-performance magnetic shield apparatus, not of the chamber type, which is applicable without difficulty for the measurement of biological magnetism such as cardiac magnetic field issued not only from a healthy person, but also from a bedridden patient.
The present inventors have therefore developed many component technologies for the purpose of developing a cylindrical-type light and high-performance magnetic shield apparatus. A magnetic shaking technology (Non-Patent Document 1); leakage inhibition of shaking magnetic field (Non-Patent Document 2); inhibition of magnetic noise caused by an external cause entering from an opening end (Non-Patent Documents 3 and 4); and further, integral forming of magnetic shield based on lamination structure using a carbon fiber reinforced plastics (CFRP) (Patent Document 1) are included.
In general, installation of a chamber-type shield apparatus afterward in the room results in a difference in floor height so that it is not easy to carry the patient, together with his or her bed, into the apparatus.
It is therefore suggested to simply divide the cylindrical shield into two and to make it movable as described in Patent Document 2. FIG. 12 illustrates an example of the separate type shield apparatus 100 having a structure in which divided pieces are arranged one to the right and the other to the left, and are joined at upper and lower positions.
According to this example, the separate type shield apparatus 100 has a structure in which at least any of two magnetic shield divided bodies 101 (101A and 101B) formed in a right-left-symmetrical shape is made movable to permit transportation of a patient together with his or her bed into the inner space.
However, the function of magnetic shield cannot be maintained with this configuration alone. Since, in the separate type shield apparatus 100, it is necessary to install SQUID gradiometers 200, and a space is formed at the joint between the two divided bodies 101A and 101B. Therefore, magnetic fluxes passing through this space (i.e., magnetic fluxes perpendicular to the joint) cannot be passed continuously, and the magnetic shielding effect fails here.    Patent Document 1: Japanese Patent Application No. 2005-80775    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-179550    Non-Patent Document 1: Ichiro SASADA, study on magnetic shaking type magnetic shield for measuring weak magnetic fields, Journal of Japan Applied Magnetism Society, 27,855-861 (2003)    Non-Patent Document 2: Nakashima Y, Kimura T, Sasada I, Magnetic field leakage from a 45° angle magnetic shell and a reduction method for a high-performance magnetic shield, IEEE Trans. on Magn. 42(10)3545-3547 (2006)    Non-Patent Document 3: Saito T, Tashiro N, Sasada I, Active compensation effect in multi-shell shield with passive shell, Journal of Japan Applied Magnetism, 29, 567-570 (2005)    Non-Patent Document 4: Umeda Y, Tashiro N, Sasada I, Application of active cancellation to cylindrical magnetic shield, Electricity Society A, 123, (8), 790-796 (2003)